Relational databases (RDBMS), such as MySQL, are used to store data, typically in one or more tables. Each table typically has one or more rows, one row for each record, for example, and one or more columns, one column for each different attribute capable of being stored.
If a given row (record) does not have a value for a column, a “null” value may be indicated for that column if the table has been set up to support storing a “null” value with the column. A database system may be configured to store null values in a space and/or computationally efficient manner, which may for example require less storage space or other resources than storing an integer “0”, a string, or any other non-null value.
For a sparsely populated table, or a table in which the same most common value is stored repeatedly for a given attribute, a single, large database table may use a lot of space to store data. In some database systems, data may be overwritten in place, i.e., in the same location(s) on storage media, resulting in unused space between database records as stored.